Various sensor systems widely are used in inspecting the nation's infrastructure of roads and bridges. Unfortunately, infrastructure integrity cannot be conveniently evaluated in an economically or time efficient way using these existing systems. Techniques using acoustic microphones, accelerometers and geophones have been proposed to meet the increasing demands for integrity evaluation and to prevent significant and potentially irreversible infrastructure degradation.
It has been widely recognized that the propagation of surface acoustic waves through concrete/asphalt layers can be measured by accelerometers or microphones; the propagation characteristics can be used to assess/estimate the material and structural properties, including the existence of damage in the surface and sub-surface of the structure. Static systems relying upon an external impact source to produce acoustic signals measureable with an acoustic transducer or an array of acoustic transducers are considered an effective way for surface and subsurface sensing.
One acoustic wave-based method for detecting the subsurface pavement profile is the air-coupled surface wave measurement using multichannel analysis of surface wave (MASW) technique. A microphone array is suspended in the air, close to the ground, to collect leaky surface waves excited by a nearby hammer impact. In the time domain, the surface wave can be separated from the direct hammer noise in subsequent data processing. After the whole set of tests are finished, an iterative dispersion analysis, relying upon manual experience and intervention, is executed in order to achieve the estimation of subsurface profile. However, the data analysis is sensitive to the interference of ambient noise and hammer noise. The manipulation from test point to test point is rather slow. Besides, an external impact excitation is not capable of measuring the surface condition.
There is a significant need for a time-efficient inspection system with an automatic excitation source. Tire and road surface interaction produces acoustic signals, and these signals are measureable with an acoustic transducer or an array of acoustic transducers. Specifically, using the tire as a mechanical excitation source eliminates the need for external impact used in previous sensor systems. Roadway surface vibrations, circumferential tire vibrations and air pressure vibrations are generated as the tire rolls over the surface of asphalt, concrete or other roadway surface. The vibrations travel as surface waves on the road and through the rubber of the tire and propagate through the air with volumetric attenuation. Acoustic waves are affected by the material and structural properties of the volume through which they propagate; therefore, by the accurate recording of acoustic wave signals and necessary signal processing, one can characterize the structural and material properties of a surface.
The current method for monitoring the static tire pressure and its change over time is the tire-pressure monitoring system (TPMS) deployed in many modern cars. This system was originally designed to identify under-inflation in any of the four tires of the vehicle. With TPMS, direct tire pressure sensors are mounted inside each tire to measure the static pressure every 30 seconds, and the information is wirelessly transmitted to the vehicle's instrument cluster. The sole purpose of the TPMS is to obtain the tire pressure and provide a low-pressure warning to the vehicle; therefore it does not provide a high sampling rate for tire pressure change, high transmitting rate, or an indication of dynamic tire pressure.
Due to a critical power requirement of electronic sensor nodes and wireless sensor networks, such as the TPMS, normal batteries are not durable enough and they typically increase size and weight of the sensor nodes. They also impose the maintenance burden of power recharging or replacement. Therefore, various energy harvesting technologies have been proposed for converting mechanical energy into electricity as an option for renewable power. Energy sources including mechanical motion, wind, ocean surface waves, and ambient vibrations have gained attention as novel green energy alternatives for powering electronic sensors.
As one of the most popular energy harvesting methods, harvesting vibration energy in either on- or off-resonance mode has been applied for wireless sensor networks and infrastructure health monitoring systems. A piezoelectric material acting as a transducer has been used in vibration energy harvesting systems, and optimum mechanical structures have been studied. These systems are usually lightweight and small in size. However, the major disadvantages of these vibration energy harvesting methods include: 1) the ambient vibration energy that the harvester can use is tiny; 2) these devices are limited/tuned for operating on single or narrow operating frequencies, thus it is impossible to adopt the same design for different applications; and 3) the overall output energy for various designs is in μW to mW range, which is insufficient for any critical instantaneous/real-time sensor network application.
Mechanical motion has been proposed as another energy harvesting method, in which energy from relative motion between an oscillating proof mass and a frame structure that is harvested. One example is harvesting kinetic energy from human working or other motion; a spring and piezoelectric material are usually used to support a proof mass in such design. However, this type of energy harvester shares the same three disadvantages as mentioned earlier for the vibration energy harvesting methods. Further, this intermittent human motion energy source is not available all the time which limits its power generation and storage capability for critical sensor node applications.
Another example is harvesting rotating kinetic energy from rotating structures based on a conventional DC motor. These DC motor based designs consist of a well-designed load mass as a gravitational torque generator, whose natural frequency is close to the rotating wheel frequency, and a rotational source on- or off-axis of the rotation of a host wheel. The amount of drag torque and tensile stress due to centrifugal force in a radial orientation depends on the speed.
The latter class of device has been tested on an instrumented rotating structure/wheel in a lab setting but are not believed to have been implemented on an actual vehicle wheel. Therefore, energy harvested by such a system cannot be used by conventional electrical sensors near a tire or within a vehicle. Further, their fixed or narrow working bandwidth and their low power density output range limits their utility in an actual vehicle.